Maneuvering of hydraulic directional valves can be done by direct controlling with, for example, hand levers in the immediate vicinity of the valve, or with remote controlling at greater or lesser distances from the directional valve. Using the latter alternative may require the use of mechanical control devices such as articulated rods, Bowden wires, etc. or hydraulic and electro-hydraulic controls. These can of course be combined in varying ways. One example of such a combination form of controlling is the maneuvering unit for a truck crane which is normally located on the truck frame behind the cab (not shown). Here the object is that the operator should be able partly to directly maneuver the directional valve by means of a hand lever in the immediate vicinity of the crane; partly to maneuver it from the opposite side of the vehicle by using a hand lever, which here would be done via an articulated rod; partly from a portable maneuvering station via, for example, electrical cables connected to a double-acting, electro-hydraulic (+EHC) control device.
The EHC control device is usually mechanically connected with the articulated rod with aid of clamps coupled to the piston rod of the control device. The latter has a double-acting operation and a proportional piston rod movement which is a function of the set controlling pressure. In order for it to lie within an appropriate area, for example between 5 and 15 bars, it is necessary to have a strong spring assembly in the control device. This spring assembly handles centering of the control device piston. In other words, the spring assembly must be dimensioned for at least the friction arising in the control device and the joints of the articulated rod, etc.
Likewise found in the directional valve are centering springs intended for overcoming flow forces and friction. These are normally dimensioned for standard performance requirements, and not for extra friction forces occurring in the control device and joints of the articulated rod, etc.
This means that a maneuvering of the directional valve with aid of hand levers--either with a hand lever directly connected to the directional valve or with a hand lever located some distance away and connected to the articulated rod--is going to require very great maneuvering force because of the fact that the spring assembly in both the directional valve and the control device must be compressed. If the EHC control device is activated, then the force does not play any role. It would however be desirable that these forces do not play any role when maneuvering is performed by means of hand levers, viz. that the hand lever force with connected, but non-activated EHC control devices, are the same as with a normal directional valve provided with only one or more hand levers.